Because a cationic electrodeposition coating is excellent in throwing power and causes little environmental pollution, it is used in a wide range of applications, such as primer coatings for automobiles. A cationic electrodeposition coating composition containing a lead compound or a chromium compound, such as lead chromate, basic lead silicate, and strontium chromate, has been heretofore proposed.
However, the use of heavy metal compounds, such as lead compounds and chromium compounds, has recently been restricted for environmental reasons, and cationic electrodeposition coating compositions that have excellent anti-corrosion properties and cause no environmental problems, without containing such heavy metal compounds but using a rust preventive pigment, have been developed and put in practical use.
For example, Patent Literature 1 discloses an electrodeposition coating method that employs multistage electrification, using an electrodeposition coating composition comprising: a resin component (A) obtained by reacting an epoxy resin (b1) having, in one molecule thereof, at least two specific epoxy-containing functional groups, and an amino compound (b2), and/or a phenolic compound (b3); a resin component (B) having a specific structural unit; a blocked polyisocyanate compound (C); and at least one rust inhibiting component (D) selected from the group consisting of metal ions selected from zirconium, titanium, cobalt, vanadium, tungsten and molybdenum, oxymetal ions of the metal, and fluorometal ions of the metal.
As another example, Patent Literature 2 discloses a method for forming a surface-treating film, which comprises applying a film-forming agent onto a metal substrate by a multistage electricity-applying system, the film-forming agent comprising: a zirconium compound; where necessary, a compound containing at least one metal, which is selected from titanium, cobalt, vanadium, tungsten, molybdenum, copper, zinc, indium, aluminum, bismuth, yttrium, lanthanide metals, alkali metals and alkaline earth metals; and a resin component.
However, the coating composition and the film-forming agent disclosed in Patent Literature 1 and Patent Literature 2, respectively, are not sufficient in terms of the amount of zirconium deposited on the substrate to be coated, and inferior in corrosion resistance, particularly, in hot salt water immersion resistance, on a non-treated steel plate.
For this reason, there has been a demand for a cationic electrodeposition coating composition that is sufficient in terms of the amount of zirconium deposited on the substrate to be coated, and excellent in corrosion resistance, particularly, in hot salt water immersion resistance, on a non-treated steel plate.